Wide-angle cameras, typically having a field of view greater than 60 degrees, often are used to capture large scenes. A wide-angle image taken by a wide-angle camera can be viewed using at least two types of projections. Linear perspective and cylindrical projections are the most typical projections used to view wide-angle images.
A linear perspective projection is an image captured by a wide-angle lens that is projected onto a flat piece of film. Linear perspective projection keeps straight lines straight at the expense of maintaining shape. This causes perspective deformations. The image would appear correct and not deformed if the viewer of the image placed their eye at the projection center. However, when viewing the wide-angle image with a smaller field-of-view the viewer expects smaller increases of image sizes as well as smaller amount of deformation on the image planes as the object rotates. This is why objects appear stretched at the edges. The larger than expected changes in size and deformation on the image plane make the user feel that the scene is not rigid, as if the scene were swimming around the viewer, particularly when viewing panoramic images.
A cylindrical projection is produced using a wide-angle camera having curved film and a rotating slit lens. Cylindrical projections are better at maintaining shape than linear projections. In addition, a cylindrical projection mitigates any apparent swimming motion. Even though the cylindrical projection is a viewing improvement over the linear projection, distortion and perception problems are still present. In particular, the cylindrical projection curves straight lines more than necessary. In addition, the cylindrical projection removes, almost completely, the illusion of turning one's head when viewing panoramic images.
Distortion and curvature problems in wide-angle images are caused by a field-of-view mismatch. In particular, due to limited viewing size on computer monitors and standard viewing distances, the angle subtended by the image when viewed is much smaller than the field-of-view of the camera that originally projected the scene to an image plane. This mismatch is the cause of many distortion and perception problems.
A related problem caused by viewing wide-angle images with a smaller field-of-view at viewing time is the misperception of depth. Wide-angle images exaggerate the depth disparity between near and far objects. One important visual cue of the depth of an object in a scene is the ratio between the image sizes of similar objects placed at near and far locations (called the depth foreshortening ratio). A smaller field-of-view results in a smaller foreshortening ratio. For example, assume a scene contains two persons standing next to each other with one person slightly further away from the camera than the other person. If the scene is captured by a regular (approximately 60 degree) field-of-view camera and the same scene is captured with a wide-angle field-of-view camera, the size of the objects in the scene will appear different. In particular, with a regular field-of-view camera, the person farther away from the camera will appear slightly further away from the camera than the other person. However, with the wide-angle field-of-view camera, the person farther away from the camera will appear much smaller than the other person. Because of this exaggerated size difference, the person farther away from the camera appears much farther away than he really is. When a wide-angle image of a deep scene (such as a video conferencing scene) is viewed on a computer monitor, the viewer's field-of-view, in general, is much smaller than the field-of-view of the actual images. Therefore, the depth perceived by the viewer is much larger than the actual depth.
One application where these distortion and perception problems manifest themselves is in video conferencing systems. Wide-angle cameras often are used in video conferencing systems to capture and transmit an image containing all participants present in a meeting room. One problem, however, is that when viewing the video conference the wide-angle image exaggerates the depth of the room. This causes the people in the middle of the image (who are usually furthest away from the camera) to appear very small compared to others in the room due to the extreme foreshortening.
These distortion and perception problems typical with wide angle camera images are even more apparent when viewing panoramic images covering a 360 degree field of view. A panoramic image can be generated by aligning and “stitching” input images that are acquired from a single camera. Alternately, a panoramic image can be created by using multiple cameras to acquire the input images and stitching them together. For example, this is the case when multiple cameras are used to generate a panoramic video. Essentially, the creation of a panoramic video entails acquiring multiple videos depicting a full 360-degree view of the surrounding scene. A camera rig, consisting of multiple cameras disposed in a back-to-back fashion, is sometimes employed for the purpose of capturing these videos. A stitching table which maps pixel positions in the input images to the pixel positions in the panoramic image is typically used to stitch the images together in the panorama. Three hundred sixty degree omni-directional cameras have been used for meeting recording and video conferencing. The camera is typically placed in the center of a table with the meeting participants sitting around the table. If the meeting table has a rectangular shape, people's head sizes on the panoramic images will vary depending on their distances to the camera. For example, the person who sits at the far end of the table will appear much smaller than the person who sits closer to the camera. Such head-size variations in the panoramic images do not look appealing visually and often it is difficult to recognize the people at the far end of the table or recognize their expressions.
Another problem with video conferencing employing panoramic images is that some omni-directional cameras generate a 3700×600 pixel panoramic image, but most users only have 1024×768 pixel displays which allow only a 1024×166 pixel panorama to be displayed. With this huge reduction in resolution, the people on the far end of the tables are not viewable without head size normalization.
Accordingly, there exists a need for a system and a method for real-time correction of images captured by a panoramic camera to alleviate distortion and perception problems associated with such images. This system and method should normalize head sizes in panoramic images and allow viewing of persons in the images with standard resolution monitors. Additionally, this system and method should be easy to set up and operate and should be computationally efficient.